[ARM] Improve non-executable support

Add support for detecting non-executable stack binaries, and adjust
permissions to prevent execution from data and stack areas.  Also,
ensure that READ_IMPLIES_EXEC is enabled for older CPUs where that
is true, and for any executable-stack binary.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

1 files changed, 79 insertions, 0 deletions

diff --git a/arch/arm/kernel/elf.c b/arch/arm/kernel/elf.c
new file mode 100644
index 000000000000..513f332f040d
--- /dev/null
+++ b/arch/arm/kernel/elf.c
@@ -0,0 +1,79 @@
+#include <linux/module.h>
+#include <linux/sched.h>
+#include <linux/personality.h>
+#include <linux/binfmts.h>
+#include <linux/elf.h>
+
+int elf_check_arch(const struct elf32_hdr *x)
+{
+	unsigned int eflags;
+
+	/* Make sure it's an ARM executable */
+	if (x->e_machine != EM_ARM)
+		return 0;
+
+	/* Make sure the entry address is reasonable */
+	if (x->e_entry & 1) {
+		if (!(elf_hwcap & HWCAP_THUMB))
+			return 0;
+	} else if (x->e_entry & 3)
+		return 0;
+
+	eflags = x->e_flags;
+	if ((eflags & EF_ARM_EABI_MASK) == EF_ARM_EABI_UNKNOWN) {
+		/* APCS26 is only allowed if the CPU supports it */
+		if ((eflags & EF_ARM_APCS_26) && !(elf_hwcap & HWCAP_26BIT))
+			return 0;
+
+		/* VFP requires the supporting code */
+		if ((eflags & EF_ARM_VFP_FLOAT) && !(elf_hwcap & HWCAP_VFP))
+			return 0;
+	}
+	return 1;
+}
+EXPORT_SYMBOL(elf_check_arch);
+
+void elf_set_personality(const struct elf32_hdr *x)
+{
+	unsigned int eflags = x->e_flags;
+	unsigned int personality = PER_LINUX_32BIT;
+
+	/*
+	 * APCS-26 is only valid for OABI executables
+	 */
+	if ((eflags & EF_ARM_EABI_MASK) == EF_ARM_EABI_UNKNOWN) {
+		if (eflags & EF_ARM_APCS_26)
+			personality = PER_LINUX;
+	}
+
+	set_personality(personality);
+
+	/*
+	 * Since the FPA coprocessor uses CP1 and CP2, and iWMMXt uses CP0
+	 * and CP1, we only enable access to the iWMMXt coprocessor if the
+	 * binary is EABI or softfloat (and thus, guaranteed not to use
+	 * FPA instructions.)
+	 */
+	if (elf_hwcap & HWCAP_IWMMXT &&
+	    eflags & (EF_ARM_EABI_MASK | EF_ARM_SOFT_FLOAT)) {
+		set_thread_flag(TIF_USING_IWMMXT);
+	} else {
+		clear_thread_flag(TIF_USING_IWMMXT);
+	}
+}
+EXPORT_SYMBOL(elf_set_personality);
+
+/*
+ * Set READ_IMPLIES_EXEC if:
+ *  - the binary requires an executable stack
+ *  - we're running on a CPU which doesn't support NX.
+ */
+int arm_elf_read_implies_exec(const struct elf32_hdr *x, int executable_stack)
+{
+	if (executable_stack != EXSTACK_ENABLE_X)
+		return 1;
+	if (cpu_architecture() <= CPU_ARCH_ARMv6)
+		return 1;
+	return 0;
+}
+EXPORT_SYMBOL(arm_elf_read_implies_exec);